Bandwidth of voice frequency signals which exist in eight bit PCM format may be reduced by converting the eight bit PCM signal to a smaller bit ADPCM (adaptive differential pulse code modulation) signal. ADPCM signals may be transmitted over the same transmission lines as PCM signal with existing PCM equipment supplemented by ADPCM encoders and decoders for converting between PCM and ADPCM signals at the ends of the transmission links. The bit size of the ADPCM signal determines the frequency or bandwidth of transmission. I often desirable to modify the bandwidth to accomodate various transmission requirements. Typical ADPCM encoder/decoder circuits are controlled by algorithms which balance and control synchronous bandwidth with quality of voice information being communicated. As a result, a particular algorithm is required by ADPCM encoder/decoder circuits for each particular synchronous bandwidth. When changing between frequencies in ADPCM transmission, the proper algorithm for a frequency must be selected by the encoder/decoder circuit to operate. Previous encoder/decoder circuits have typically used distinct control signals to select the correct algorithm for a predetermined frequency of operation. Additional control signals require additional circuit inputs or I.C. pins to be included in the fabrication and packaging of the encoder/decoder circuit. An algorithm control signal is typically required at both an input terminal which receives eight bit PCM data to be encoded and at an input terminal which receives ADPCM data to be decoded. Therefore, additional packaging inputs are required for an encode/decode circuit.